Growing UV LEDs: KAUST’s perfect epitaxy recipe
In a recent paper published in the Nanoscale journal under the title “Self-planarized quantum-disks nanowires ultraviolet-B emitter utilizing pendeo-epitaxy”, researchers from King Abdullah University of Science and Technology (KAUST) explain how they came to get the equivalent of a nearly defect-free planar structure by growing dislocation-free AlGaN-based nanowires nucleated so close together that they end up coalescing at their upper extremities.
Through the precise control of the initial GaN nucleation process on a clean silicon wafer, and following specific growth conditions using catalyst-free plasma assisted molecular beam epitaxy (PAMBE), they claim to have overcome many of the issues surrounding nanowire growth, obtaining high-density, spontaneously coalesced, dislocation-free AlGaN NWs with a uniform height, meaning they present a self-planarized top surface. About 30nm in diameter at their base, the inverse-tapered nanowires expanded as they grew to a diameter of roughly 70nm at the top of their 400nm length.
The nanowires were grown to integrate 15 pairs of AlxGa1-xN/AlyGa1-yN Qdisks occupying nearly half the volume of the entire NW, together with a thin p-type AlGaN electron-blocking layer. A p-type graded AlGaN layer (about 25nm thick) and a p-GaN layer (about 20nm) were grown as a top contacting layer.
Because of their high density (roughly 1×1010cm-2), their coalescence and their uniform height distribution over a large area, the nanowires reached a fill factor over 95% and Ni/Au layers could directly be deposited on top of the coalesced p-GaN layer without prior planarization.
The researchers characterized the emission of such large-area AlGaN Qdisks-in-NWs LEDs, measuring a 303nm electroluminescence at room temperature with a narrow linewidth emission spectrum (for a 48A/cm2 bias or 120mA over an area of 0.5×0.5mm2) with a full width at half maximum (FWHM) of about 20nm. Turn-on voltage was 9V.
In all, the researchers conclude to have established the simplest fabrication method for efficient UV LEDs, without using a pre-patterned substrate or special mask and removing the need for UV absorbing polymer materials for planarization (since there is no planarization step). What’s more the nanowires feature a large gain medium (active region/nanowire length-ratio over 50%), with 15-stack Qdisks efficiently combining the majority of the injected carriers and enhancing carrier recombination through their self-planarized coalescence at the top.
Another benefit of these quantum-disk nanowires grown on a low-cost and scalable silicon substrate is their improved carrier injection using graded AlGaN layers. The researchers hope their approach could be adopted for developing other type of nanowire-based devices.
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